Poly(epsilon-caprolactone) (PCL) and poly(l-lactide) (PLA) were prepared by ring-opening polymerization (ROP) using samarium(III) acetate (SmAc3) and amino acid complexes of samarium(III) as initiators. The catalytic behavior of samarium(III) acetate and their respective amino acid complexes, Sm(2,2'-bipyridine)(L-n)(3) (L-1 = l-aspartic acid; L-2 = l-glutamic acid and L-3 = glycine), was studied. It could be observed that the amino acid structure and chemical bond type have great influence on the catalytic activity. Polymerization reaction temperature was 125 A degrees C and the effect of time reaction on the conversion of monomers to polymers and the molecular weight were studied. The results indicate that the initiators induce the polymerization of epsilon-caprolactone (epsilon-CL) and l-lactide (l-LA). Polymers were characterized by size exclusion chromatography (SEC) and nuclear magnetic resonance (H-1-NMR) and size exclusion chromatography (SEC). Polyesters with average-number molar mass of 1.50 x 10(3) - 10(4) Da were obtained. Based on the H-1-NMR end group analysis of low molecular weight of polymers, a coordination-insertion mechanism is proposed for the polymerization of lactones. Kinetics study indicated a polymerization rate of first order with respect to monomer concentration. Differences in the rates of polymerization in the four initiators appeared to be governed by the different degrees of steric hindrance in the initiator structure and monomer.